The present invention relates to an optical fiber amplifier. More particularly, it relates to an optical fiber amplifier for providing large optical output power resulting from pump light generated by wavelength multiplexing. TECHNICAL REPORT OF IEICE (EDM 96-39, CPM 96-62, OPE 96-61, LQE 96-63) describes a conventional optical fiber amplifier whose optical output power is increased by multiplexing pump light. This conventional optical fiber amplifier will now be described with reference to FIGS. 19 and 20.
As is shown in FIG. 19, an erbium-doped fiber (hereinafter referred to as the EDF) 33 is connected, at its input, with an input optical fiber 31 for transferring signal light with an isolator 32 interposed therebetween, and is also connected, at its output, with an output photometric fiber 35 with an isolator 34 interposed therebetween. The input and the output of the EDF 33 are also connected with first and second wavelength division multiplexer (hereinafter referred to as the WDM) couplers 36 and 37, respectively. Each of the WDM couplers 36 and 37 is interposed in order to couple the signal light in a 1.55 .mu.m band with pump light in a 1.48 .mu.m band, which is one of the absorption wavelength bands of the EDF 33, with a small loss.
In this optical fiber amplifier, eight semiconductor lasers 1 are used as pump light source for the EDF 33. Among the eight semiconductor lasers 1 for optical pumping, four have an oscillation wavelength of 1.465 .mu.m and the other four have an oscillation wavelength of 1.485 .mu.m, and four are disposed at the input side of the EDF 33 and the other four are disposed at the output side of the EDF 33. Among the four lasers disposed at the input side, two have an oscillation wavelength of 1.465 .mu.m and the other two have an oscillation wavelength of 1.485 .mu.m. Output light of the two lasers having the same oscillation wavelength are coupled by a polarization coupler 41a, 41b, 41c or 41d. By adopting the polarization coupling, pump light in a TE mode and pump light in a TM mode having the same wavelength can be coupled without causing interference, resulting in decreasing a light loss.
The pump light with a wavelength of 1.465 .mu.m and the pump light with a wavelength of 1.485 .mu.m obtained by the polarization coupling are multiplexed by a wavelength multiplex coupler 36a or 37a, and then allowed to enter the EDF 33 by the first or second WDM coupler 36 or 37. FIG. 20 shows a spectrum of the wavelength-multiplexed pump light and a transmission wavelength band characteristic of the wavelength multiplex coupler 36a (37a).
In this manner, the optical fiber amplifier of FIG. 19 amplifies an optical signal input through the input optical fiber 31 by allowing the pump light from the four pumping lasers at the input side and the pump light from the other four pumping lasers at the output side to enter the EFD 33 in the two directions. Such a conventional optical fiber amplifier, however, cannot always provide output light with sufficiently large optical power.